The increasing demand for environmentally friendly, sustainable and renewable energy sources is driving the development of large area, thin film photovoltaic (TFPV) devices. With a long-term goal of providing a significant percentage of global energy demand, there is a concomitant need for Earth-abundant, high conversion efficiency materials for use in photovoltaic devices. The categorization of earth abundant elements can be described in various ways, for example, by crustal abundance, relative annual commercial production, or price. For example, by crustal abundance, the top 30 elements are O, Si, Al, Fe, Ca, Mg, Na, K, Ti, C, H, Mn, P, F, S, Sr, Ba, V, Cl, Cr, Zr, Ni, Zn, Cu, Rb, Ce, La, Nd, Co, Y. See, e.g., Alharbi, F., et al. “Abundant non-toxic materials for thin film solar cells: Alternative to conventional materials” 2011 Renewable Energy 36, Issue 10, October 2011, Pages 2753-2758. (O and N are available from the atmosphere.) A number of Earth abundant, direct-bandgap semiconductor materials now seem to show evidence of the potential for both high efficiency and low cost in Very Large Scale (VLS) production (e.g., greater than 100 GW), yet their development and characterization remains difficult because of the complexity of the materials systems involved.
Among the TFPV technologies, CuInxGa1−xSe2 (CIGS) and CdTe are the two that have reached volume production with greater than 11% stabilized module efficiencies. However, the supply of In, Ga and Te may impact annual production of CIGS and CdTe solar panels. Moreover, price increases and supply constraints in Ga and In could result from the aggregate demand for these materials used in flat panel displays (FPDs) and light-emitting diodes (LEDs) along with CIGS TFPV. Also, there are concerns about the toxicity of Cd throughout the lifecycle of the CdTe TFPV solar modules. Efforts to develop devices that leverage manufacturing and R&D infrastructure related to these TFPV technologies but using more widely available and more environmentally friendly materials should be considered a top priority for research. The knowledge and infrastructure developed around CdTe and CIGS TFPV technologies can be leveraged to allow faster adoption of new TFPV materials systems.
Optical absorbers for use with solar cells are more economically attractive if they have high efficiency and can be made from earth-abundant materials that are available at low cost. CZTS absorbers are being widely studied to meet these performance goals. CZTS has a strong absorption coefficient for visible light making it possible to use thinner absorber layers further reducing costs of assembled solar cells. CZTS is comprised of Cu, Zn, Sn, and Se. In both CIGS and CZTS materials, S can be substituted for some or all of the Se.
It is also possible to make similar absorbers using various other combinations of earth-abundant materials, but these have not yet been developed as extensively, and their performance potential is not yet known. One such example material is Fe2(Si,Ge)(S,Se)4 which is described in commonly owned, co-pending U.S. patent application Ser. No. 13/727,986, incorporated herein by reference. Another material that has been described is Cu2SnS3 for which some preliminary studies have been reported by Devendra et al. (“Direct Liquid-Coated Cu2SnS3 as a New Absorber Material for Thin-Film Solar Cell,” 38th IEEE PVSC, 2012), Berg, et al. (“Thin film solar cells based on the ternary compound Cu2SnS3,” Thin Solid Films, 520, 6291-94, 2012), Bouaziz et al. (“Growth of Cu2SnS3 thin films by solid reaction under sulphur atmosphere,” Vacuum, 85, 783-86, 2011), and Fernandes et al. (“A study of ternary Cu2SnS3 and Cu3SnS4 thin films prepared by sulfurizing stacked metal precursors,” J. Phys. D: Appl. Phys., 43, 215403, 2010). The use of Zn3P2 as a solar cell absorber has been described in U.S. Pat. Nos. 4,342,879 and 4,477,688, and power conversion efficiencies of 4-6% have been reached. In addition, BaSi2 is a semiconductor with a bandgap of 1.4 eV, the use of which as a solar absorber has been described in U.S. Patent Application Publication Nos. 2010/0252097 and 2012/0049150.
The development of TFPV devices exploiting Earth abundant materials represents a daunting challenge in terms of the time-to-commercialization. That same development also suggests an enticing opportunity for breakthrough discoveries. A quaternary system such as CIGS requires management of multiple kinetic pathways, thermodynamic phase equilibrium considerations, defect chemistries, and interfacial control. The vast phase-space to be managed includes process parameters, source material choices, compositions, and overall integration schemes. The complexity of the intrinsically-doped, self-compensating, multinary, polycrystalline, queue-time-sensitive, thin-film absorber (CIGS), and its interfaces to up-, and down-stream processing, combined with the lack of knowledge on a device level to address efficiency losses effectively, makes it a highly empirical material system. The performance of any thin-film, (opto-)electronically-active device is extremely sensitive to its interfaces, especially when contacting dissimilar materials where at least one of the materials is a multinary compound semiconductor. Interface engineering for electronically-active devices is highly empirical. Traditional R&D methods are ill-equipped to address such complexity, and the traditionally slow pace of R&D could limit any new material from reaching industrial relevance when having to compete with the incrementally improving performance of already established TFPV fabrication lines, and continuously decreasing panel prices for more traditional cSi PV technologies.
Due to the complexity of the material, cell structure, and manufacturing process, both a complete fundamental scientific understanding and large scale manufacturability are yet to be realized for TFPV devices. As the photovoltaic industry pushes to achieve grid parity, much faster and broader investigation is needed to explore the material, device, and process windows for higher efficiency and a lower cost of manufacturing process. Efficient methods for forming different types of TFPV devices that can be evaluated are necessary.